1. Field of the Invention
The invention relates to lamps that provide good color rendition of a wide range of colors, while having much greater efficiency than an incandescent lamp; and more particularly, to high intensity discharge lamps having metal halide additives in the arc tube.
The color characteristics of light sources have become increasingly important as architects and designers use light as an element in the design of a room or a structure. In the past, good color rendition, lamp-to-lamp color uniformity and color shift over the life of the lamp, and a desirably low correlated color temperature have dictated the use of either tungsten-halogen lamps or certain deluxe phosphor-coated mercury HID lamps. However, in circumstances where high efficiency is desired, or a more compact source than the typical fluorescent tube are desired, neither of these lamp sources have proved to be entirely satisfactory. To fill this gap, there has been interest and experimentation in the development of metal halide high intensity discharge lamps for over two decades. However, certain disadvantages of these lamps have limited their use in critical applications.
2. Description of the Prior Art
Early sodium iodide/scandium triiodide lamps, such as described in U.S. Pat. No. 3,407,327, while having better color properties than simple metal vapor arcs, were still not able to provide the desired color rendition, particularly in the red region, and had undesirably high color temperature, lamp-to-lamp color variation, and color shift over the lamp life. Lamps using a tin halide additive have been shown to provide excellent color rendering properties, but have an undesirably high color temperature where the tin halide is the only significant additives.
A promising attempt to overcome these disadvantages is described in U.S. Pat. No. 4,360,758, hereby incorporated by reference, which describes a calcium/thallium/tin iodide discharge which provides excellent color rendering properties and control of the correlated color temperature. However, in order to obtain sufficient vapor pressure from the calcium iodide component of the filling, so as to have good red emission, it is necessary to operate the lamp so that the arc tube has a cold spot temperature which is greater than 750.degree. C. This has been achieved by overwattage operation, which increases the wall loading of the arc tube, or requires the use of formed niobium or nickel end caps to elevate the cold spot temperature of the arc tube.
Another approach to the improvement of color rendering has involved the use of short arc lengths with relatively high vapor pressure. Metal halide lamps of this sort are described in an article by Fromm, Seehawer, and Wagner in Lighting Research and Technology 11,1 (1979). Of particular interest in this article is the comparison of the temperature distribution of a conventional long arc metal halide lamp, and a lamp whose arc tube has bell-shaped ends so as to achieve a more nearly isothermal temperature distribution along the arc tube. An alternative design described in that article involves a rounded arc tube used for a short arc 250 watt metal halide lamp, which showed a temperature variation between cold spot and hot spot of only 80.degree. C. in the horizontal position, and 85.degree. C. in the vertical position. However, when data for this lamp in both vertical and horizontal operation are superimposed, it becomes clear that there is a far greater total variation, and that the cold spot temperature is much too low if the additives include a material such as calcium iodide. Thus, the isothermal effect desired is not achieved by the configuration shown, if a lamp is to be useable in both orientations.
A further problem encountered in the manufacture of high color rendering metal halide lamps is related to the hygroscopic properties of the additives. The hygroscopic nature of sodium iodide salts has required the development of arc tube filling and processing techniques by which the tube may be repeatedly evacuated and flushed, and care is exercised that the additive salts be protected from moisture contamination. Experimental use of an additive fill such as that taught in the U.S. Pat. No. 4,360,758 has, however, shown excessive difficulty in starting because of the even greater hygroscopic nature of the calcium iodide additive. This problem has not proved solvable by any of the known techniques that are useable in manufacture of production quantities.
For example, one in situ purification method involves "torching." After an arc tube has been flushed, and additives have been added, the arc tube is locally heated at the location of the dosed additives, so that they evaporate and condense elsewhere. This may be performed while the tube is evacuated or contains argon. Before the tube has cooled off, it is again evacuated, then cooled, then given the final filling, and is tipped off (tubulation sealed off). During this process some of the additives are drawn off into the evacuation system, so that lamp-to-lamp non-uniformities result. Further, the iodide additives used are corrosive and damaging to the vacuum system.